Modern imaging methods are frequently used to generate two- or three-dimensional image data that can be used visualize an imaged object under examination and in addition to this also for further applications.
Imaging methods are frequently based on the acquisition of X-rays wherein so-called projection-measurement data is generated. For example, projection-measurement data can be acquired with the aid of a computed tomography system (CT system). Typically with CT systems, a combination of an X-ray source and an oppositely disposed X-ray detector arranged on a gantry rotates about a scanning chamber in which the object under examination (hereinafter, called the patient without restricting the generality) is located. In this context, the center of rotation (also called the “isocenter”) coincides with a so-called system axis z. The patient is irradiated with X-rays from the X-ray source in the course of one or more rotations, wherein the oppositely disposed X-ray detector acquires projection-measurement data or X-ray projection data.
The X-ray detectors used in CT imaging usually comprise a plurality of detection units, which are generally arranged in the form of a regular pixel array. The detection units each generate a detection signal for X-rays striking the detection units said signal being analyzed at defined time points with respect to the intensity and spectral distribution of the X-rays in order to obtain information on the object under examination and generate projection-measurement data.
One field of application of computed tomography is the examination of coronary vessels. With coronary CT-angiography, the patient is first given a contrast medium. This is following by a depiction of the coronary vessels. The images can be used to determine the condition of the coronary vessels, wherein it is possible to check whether a patient is at risk of myocardial infarction or not. However, the administration of contrast media and the irradiation of the patient with X-rays are stressful. In particular, the administration of contrast media can be contraindicated in the case of restricted renal function.
A further method for examining the coronary vessels is known as calcium scoring. With this method, a CT image is taken without the use of a contrast medium and with low radiation exposure. On the basis of the density of the calcification, the calcium content of the coronary vessels is rated on a scale, the so-called Agatston score, and assigned to one of four categories (0-10; 10-100; 100-400; >400). The level of the calcium content is an indicator of the probability of myocardial infarction or at least angina pectoris developing within the next few years. Therefore, from a statistical viewpoint, a calcium content of 0-10 means that there is a low risk of the patient suffering myocardial infarction in the next five years. On the other hand, a calcium content of more than 400 means that the patient is at increased risk of myocardial infarction.
If the Agatston score is more than 500-600, it may be advisable additionally to perform a CT angiography scan in order to obtain a more accurate picture of the condition of the coronary vessels. The classification of such values relating to the calcium content with respect to reference values requires the observance of rigidly prespecified CT-acquisition parameters, in particular in respect of the required X-ray spectrum or the electric X-ray tube voltage correlated therewith. However, even the choice of standardized parameters does not eliminate certain uncertainties with the classification since different types of prefiltering with different CT systems means relatively high fluctuations or tolerance are to be expected (typically 6% for the Agatston score and 8% for the volume score).
In addition, the definition of the spectrum (corresponding to a tube voltage of 120 kV) for example for the Agatston score is based on historical factors. This value is related to the limited power reserves of an electron beam CT system, but is in no way ideal with respect to the dose efficiency of the calcium depiction. Low-energy spectra are much more efficient, but change the value of the Agatston score significantly and hence impede comparability with reference values.
For the determination of the Agatston score, a region to be depicted is divided into three-millimeter-thick image slices to be depicted. In each of the image slices, calcification is determined in that account is taken of regions with attenuation values of more than 130 HU. However, in this case, regions with an area of less than 1 mm2 are ignored in order to suppress image noise. Each level of calcification detected is assigned to a region designated an ROI (region of interest) and a maximum attenuation value CTmax is determined in the respective region. Then, each of the regions is assigned a weighting factor wi, which is a function of the maximum attenuation value CTmax determined. The weighting factor wi for each region ROIi is calculated as follows:
                              w          i                =                  {                                                                                          1                    ,                                                                  if                        ⁢                                                                                                  ⁢                        130                        ⁢                        HU                                            ≤                                              CT                        max                                            ≤                                              200                        ⁢                        HU                                                                                                                                                              2                    ,                                                                  if                        ⁢                                                                                                  ⁢                        200                        ⁢                        HU                                            ≤                                              CT                        max                                            ≤                                              300                        ⁢                        HU                                                                                                                                                              3                    ,                                                                  if                        ⁢                                                                                                  ⁢                        300                        ⁢                        HU                                            ≤                                              CT                        max                                            ≤                                              400                        ⁢                        HU                                                                                                                                                              4                    ,                                                                  if                        ⁢                                                                                                  ⁢                        400                        ⁢                        HU                                            ≤                                              CT                        max                                                                                                                  .                                              (        1        )            
The calcium score CSi assigned to a region ROIi is then obtained asSCi=wi·Ai,  (2)wherein Ai identifies the area of the respective region ROIi.
It is also possible for a total value CS, hereinafter a value called the Agatston score, to be determined from the respective calcium score CSi as follows:
                    CS        =                              ∑            i                    ⁢                                    CS              i                        .                                              (        3        )            
A further value used for the Ca scoring is the so-called volume score. The volume score provides information regarding the calcification volume. For the determination of the volume score, the number of voxels exceeding a threshold value is multiplied by the respective voxel volume, wherein a technique of isotropic interpolation is used. Here, the threshold value used is preferably a value of 130 HU.
Another value used for the calcium scoring is the so-called mass score. The mass score provides information on the total mass of the calcification identified.
When reference is made to the determination of a calcium score is the following, this term covers the above-named parameters: Agatston score, volume score and mass score.
It would be desirable to adapt the protocol parameters for a CT image for the determination of a calcium score in deviation from the defined standard parameters, for example an X-ray tube voltage of 120 kV, to the respective requirements with respect to image quality or dose reduction.